1. Field of the Invention
The present invention relates to a process for preparing a thermally shrinkable multilayer film and, more particularly, to a process for preparing a thermally shrinkable multilayer film suitable, in particular, for packing processed meat such as roast pork or cuts of fresh meat.
2. Description of the Prior Art
Processed meat such as roast pork and so on had heretofore been processed and sold by individual meat shops; however, processed meat is now being produced and distributed by large meat processors and food manufacturers to individual meat shops because storage methods for a thermally shrinking package with oxygen barrier properties have been developed and have made it possible to store such processed meat for a long period of time. And fresh beef meat and so on had heretofore been transported without cutting from butcheries directly to individual meat shops. Recently, however, fresh meat is cut into pieces at butcheries and transported to individual meat shops as cut meat while being refrigerated at temperatures around 0.degree. C. after being packed in a thermally shrinked manner.
As films for thermally shrinkable packaged processed meat and cut meat there have heretofore been preferably employed thermally shrinkable multilayer films containing a polyamide resin layer from the viewpoint of mechanical strength. In order to store packed contents for a longer period of time, there has preferably been employed a thermally shrinkable multilayer film with an oxygen barrier resin layer composed of a saponified ethylene-vinyl acetate copolymer (heretofore referred to as "EVOH") as a middle layer. The thermally shrinkable multilayer films have been prepared by laminating a sealant material such as a polyethylene resin film or an ethylene-vinyl acetate copolymer resin film or an EVOH film to a thermally shrinkable polyamide resin film by means of a dry laminating method. From the viewpoint of productivity and thermal shrinkability, it is preferred to use a coextrusion laminating orientation method for producing the thermally shrinkable multilayer film; however, such thermally shrinkable multilayer films which are produced by a coextrusion laminating orientation method, have the disadvantage that they may cause curling or be transformed because they have a natural shrinkability or the film flatness may be impaired. Accordingly, they had to be produced without causing curls and a natural shrinkability as well as be set so as to cause little decrease in a thermal shrinkability. Heretofore, there has been employed a process that sets such thermally shrinkable multilayer film by heating so as to lead to a thermal shrinkage while providing the resulting film with a relaxation.
The thermally shrinkable multilayer film with a polyamide resin layer as an outermost layer on one of the surface sides has generally caused a large degree of curling. Accordingly, such a polyamide layer has often been employed, for example, as a middle layer while the both outermost layers have been composed of an identical resin.
In conventional processes for producing thermally shrinkable multilayer films where there are employed the conventional setting procedures that involve heating coextrusion laminated, oriented films or shrinking thermally while giving a relaxation, if the setting were carried out so as not to cause any curling or natural shrinkability, thermally shrinkable characteristics are impaired to a large extent. If the setting were carried out so as to maintain a thermal shrinkability inherent in thermally shrinkable multilayer films, curling and natural shrinkability generally result. In particular, the thermally shrinkable multilayer film with a polyamide resin layer laminated on one of the surfaces as an outermost layer is composed of a polyamide resin capable of being set at temperatures different to a considerable extent from temperatures at which a resin to be employed for a sealant material or the like is set so that the tendency as described hereinabove is rendered all the more remarkable. More specifically, in instances where the thermally shrinkable multilayer film is set at high temperatures at which no natural shrinkability of the polyamide resin layer is caused to occur and/or at high rates of relaxation, on the one hand, the sealant material layer is caused to lose its thermal shrinkability. In instances where the thermally shrinkable multilayer film is set at low temperatures at which the sealant material layer and so on can maintain its thermal shrinkability and/or at low rates of relaxation, on the other hand, an amount of natural shrinkability of the polyamide resin layer is caused to be increased to a large extent. As a result, the resultant thermally shrinkable multilayer film has a high extent of natural shrinkability, thereby leading to the impairment of flatness of the film or causing curls.